See:
"World-Wide Web Proxies" (I think this paper introduced the protocol)
http://www.w3.org/hypertext/WWW/Proxies/
"CERN server as a Proxy"
http://www.w3.org/hypertext/WWW/Daemon/User/Proxies/Proxies.html
"Caching Algorithm of CERN httpd"
http://www.w3.org/hypertext/WWW/Proxies/Caching/Overview.html
(I think there were some more papers at the first and second WWW conferences)
Here's the write-up on the Expires Header:
>The Expires field gives the date and time after which the information
>given ceases
>to be valid and should be retrieved again if it has been kept as a local
>copy. This
>allows control of caching mechanisms, but the date and time indicated does not
>necessarily imply that the original object will cease to exist. This is
>completely
>controlled by the server. The format is an absolute date and time as defined by
>HTTP-date in Section 4.1. [...]
>Note
>This field can also be used for automatic refreshing of dynamic or
>volatile data.
>However, this is completely dependent on the implementation of the client
>application to automatically issue a new request when the object has expired.
>
>Object's generated by data-producing processes, or scripts which can be run by
>such processes, are often dynamic by nature. Therefore, it is strongly
>recommended that responses containing such Objects's contain an Expires header
>field.
See also the write-ups on Last-Modified, If-Modified-Since, and Pragma: in
the current HTTP draft pointed to at:
http://www.ics.uci.edu/pub/ietf/http/
My general advice is not to panic about the "what ifs" till you know more
about the protocol.
--- Albert Lunde Albert-Lunde@nwu.edu